1. Field of the Invention
The present invention relates to a digital displacement measuring instrument for measuring a dimension or the like of an object to be measured based on a displacement of a spindle.
2. Description of Related Art
A digital micrometer using a lead screw has been known as one of digital displacement measuring instruments.
A digital micrometer includes a body, a spindle that is screwed into the body to be axially moved, an encoder that detects a displacement of the spindle, and a display that digitally displays the displacement of the spindle detected by the encoder.
In a typical digital micrometer, a measuring force is controlled at a constant level is used for reducing variance of measurement values.
For example, there has been known a micrometer that includes a rotatable operation sleeve at an outer end of the spindle and a constant pressure mechanism (e.g., a ratchet mechanism) interposed between the spindle and the operation sleeve. In the micrometer, when a predetermined load or more is applied on the spindle, the constant pressure mechanism is actuated to idle the operation sleeve, thereby controlling a measuring pressure at a constant level (see Patent Literature 1: JP-A-2001-141402).
However, in the above-mentioned micrometer, even using the constant pressure mechanism in order to control the measuring force at a constant level, measurement results vary depending on a moving speed of the spindle when the spindle is brought into contact with an object to be measured. This is because, in an arrangement where the spindle is screwed into the body, threaded portions of the spindle and the body easily bite each other by a wedge effect. Particularly, since a biting amount differs depending on the moving speed of the spindle when the spindle is brought into contact with the object, the measurement results vary.
Recently, in response to a demand for high efficiency, a lead of a threaded portion of the spindle is configured to be large. Since the moving speed of the spindle becomes higher in this configuration, the measurement results are more likely to vary.